|Pilot study on the influence of feeding conditions at the North Sea on the breeding results of the Sandwich Tern Sterna sandvicensis|
Brenninkmeijer, A.; Stienen, E.W.M. (1994). Pilot study on the influence of feeding conditions at the North Sea on the breeding results of the Sandwich Tern Sterna sandvicensis. IBN Research Report, 94(10). Institute for Forestry and Nature Research (IBN-DLO): Wageningen. 51 pp.
Part of: IBN Research Report. Institute for Forestry and Nature Research (IBN-DLO)/Instituut voor Bos- en Natuuronderzoek: Wageningen. ISSN 0928-6896, more
|Authors|| || Top |
- Brenninkmeijer, A., more
- Stienen, E.W.M., more
Of all gulls and terns breeding along the Dutch coast the Sandwich Tern is the most specialised piscivorous bird. During the breeding season terns feed mainly on pelagic fish as herring, sprat, sandeel, and lesser sandeel. This makes the species extremely vulnerable to changes in the marine ecosystem.During the twentieth century, the number of breeding Sandwich Terns in the Netherlands has shown large fluctuations. In the beginning of this century, numbers were low, because of large-scale egg-collecting and shooting of adults. As a result of the protection following the slaughter for the lady's hat fashion, numbers gradually increased up to 45,000 pairs in 1938. In World War II, egging again caused a drop in numbers. After World War 11, the population increased up to 36,000 pairs in the 1950s. At the end of that decade, numbers dropped markedly and the number of breeding pairs were reduced to 875 in 1965. This decrease was due to a pollution of the Dutch coastal waters by organochlorine pesticides. After the pollution stopped, the population increased in numbers again, but the large numbers from the period previous to the pollution have never been reached since then. Compared to previous population recoveries, the recovery after the pollution in the 1960s was slow, and since the 1 980s the population seems to stabilise at approximately 11,000 breeding pairs, almost one third of the population during the 1930s and the 1950s.This leads to the main subject of this report: 'What is the reason of the slow recovery of the Sandwich Tern after the numerical decline in the 1960s and why is there a stabilisation of breeding pairs at such a low level?’. Although this could be due to many factors, we have focussed our study on the food situation during the chick rearing period, The study deals with three items:( 1 ) What is the species composition and what is the length distribution of prey items delivered to the chicks of Sandwich Terns on Griend, the main colony in The Netherlands, and have there been remarkable shifts compared with the late 1960s and early 1970s (chapter 2)?(2) Where do the terns feed, and are there any shifts in this respect compared with the late 1 960s and early 1970s (chapter 3)?(3) Is there a relationship between fish availability and either population size or breeding success of the Sandwich Tern (chapter 4)?The species composition of the fish delivered to the chicks in 1992-1993 was identical to that in 1969-1974. The number of fish brougth to the chicks, however differed markedly from that in 1969-1974. In bath years (mast pronounced in 1992) the food supply was law compared with 1969-1974, and also compared with foreign colonies. The amount of food actually eaten by the chicks was strongly influenced by windspeed. An increasing windspeed did not only lower the supply of food (resulting from a lower foraging success) but also increased kleptoparasitism by Black- headed Gulls. The low food supply in recent times might be a possible explanation for the stabilisation of the number of breeding pairs since the early 19805.Experiments with radio-tagged adults revealed that the foraging area of the terns is not markedly different from that in 1970, although there a little shift towards the Wadden Sea.Analyses of data on the abundance of young herring in the North Sea and the number of breeding Sandwich Terns revealed a positive relationship between these two parameters. However, the number of fledged young showed no relationship with the abundance of young herring. Although the data set on young herring refers to the total North Sea and do not necessarily show the local abundance of this species, the results suggest that the breeding population of the Sandwich Tern at Griend is regulated by the availability of food.